Conventionally, for packaging various kinds of foods such as jelly and pudding, a packaging body including a container and a lid has been generally used. The container used for the packaging body is thermoformed, and after the container is filled with food or the like, the lid is heat-sealed (welded) to a flange extending outward from a circumference of an opening of the container.
The packaging body is preferably heat-sealed firmly in terms of keeping quality of a content and transportation (sealing performance). However, in terms of usability in opening the lid to use the content, the lid is desired to be easily opened (easy-open performance). Therefore, there has been a demand for a packaging body satisfying the sealing performance and the easy-open performance as contradictory performances described above.
In order to satisfy such demand, there has been suggested a method as shown in FIG. 9, where a container 100 is formed from a multilayer sheet and an innermost layer 100A of a flange 101 and a lid 7 are heat-sealed to each other, then when the lid 7 is peeled off, layer peeling is generated between the innermost layer 100A and an adjacent layer 100B adjacent to the innermost layer 100A to cause the innermost layer 100A to be peeled off with the lid 7 (see, for instance, JP-B-5-67509, pages 1 to 4, FIGS. 1 and 2, etc.).
Although the method is an excellent packaging method, it becomes difficult to peel off the innermost layer 100A by layer peeling in some cases, where the heat-sealing is performed with high temperature and high pressure to enhance heat-sealing strength between the lid 7 and the innermost layer 100A, and a seal resin is melted and flowed to an end surface of the flange 101 depending on resin types of the inner most layer 100A and the lid 7, melt viscosities of the resins, etc., causing the seal resin to cover an edge of a peeled surface H formed on the adjacent later 100B.
Therefore, a heat-seal condition has to be controlled in a narrow range to obtain sufficient sealing performance and easy-open performance.